1. Field of the Invention
The disclosure relates to memories. More particularly, and the disclosure relates to memories and write current improving techniques thereof.
2. Description of the Related Art
Today, there are several current-controlled memories. The physical properties of a memory cell can be controlled by the write current flowing therethrough. The data stored in the memory cell is dependent on the physical properties thereof. Magnetoresistive Random Access Memory (MRAM) and Phase Change Memory (PCM) are current-controlled memories. FIG. 1 depicts the essential structure of a conventional MRAM. As shown, a first voltage source 102, a second voltage source 104, a bit line (depicted by its equivalent resistor 106), write switches 108 and 110, and an address decoder 112 are included. The circuit represents one memory cell. The data stored in the memory cell is dependent on the magnetization of the memory cell, and the magnetization of the memory cell is dependent on a write current direction on the bit line (resistor 106). Logic ‘0’ may be represented by one magnetization direction, while logic ‘1’ may be represented by the other magnetization direction.
This paragraph discusses the operations of the circuit of FIG. 1. An address signal 114 is sent to the address decoder 112 to be decoded to a bit line select signal 116. When the bit line select signal 116 is enabled, the write switches 108 and 110 are turned on to allow current to flow through the bit line (resistor 106). The first and second voltage sources 102 and 104 are designed to provide voltages in a write period after the enabling of the bit line select signal 116. In the write period, a write current, from the first voltage source 102 to the second voltage source 104 or from the second voltage source 104 to the first voltage source 102, is provided on the bit line (resistor 106). The two different current directions allow the memory cell to store two different logic levels.
The direction of the write current is dependent on the voltage levels provided by the voltage sources 102 and 104. When the first voltage source 102 provides a high voltage level and the second voltage source 104 provides a low voltage level (such as a ground), the write current flows from the first voltage source 102 to the second voltage source 104. On the contrary, when the first voltage source 102 is the ground and the second voltage source 104 provides the high voltage level, the write current flows from the second voltage source 104 to the first voltage source 102.
An MRAM is a non-volatile memory and has high memory cell density, fast read/write speeds and high radiation resistance. However, one drawback of MRAMs is that high write currents are required. In conventional techniques, the write switches 108 and 110 are realized by MOSFETs. The maximum current provided by an MOSFET is limited to the saturation current thereof. To provide enough write current, conventional techniques usually increase the size of the MOSFETs or minimize the resistance of the bit line (resistor 106).
The conventional techniques result in large-sized memories and complex memory structures, thus increasing the costs for the memory.